It is known in the prior art to carry telephone calls between local telephone operating companies through the AT&T network or through one or more independent inter-exchange carriers such as MCI or Sprint. The local telephone operating companies operate within a so-called local access and transport area (LATA). When a long distance call is dialed, the call is usually transmitted through an operating company central office to a point of termination in the originating LATA at which it is picked up by the inter-exchange carrier and passed by that carrier on to a termination point in a distant LATA. Upon reaching the destination LATA, the call is then transferred by the inter-exchange carrier to the local operating company central office within that LATA for ultimate connection to the original called station therein. Typically, the termination points of each LATA include suitable switching circuits, e.g., an access tandem, that are interconnected by a digital serial link. Such digital links are also presently used to interconnect virtually all central offices as well as to interconnect operating company switching networks to one or more cell site control switches of a mobile telephone network.
It is also known in the prior art to provide "automatic voice messaging" where, upon the occurrence of a busy/ring-no-answer condition at a called station, the user of the calling station can be connected to a voice message facility for recording a voice message for subsequent delivery to the called station. Automatic voice messaging has not heretofore been compatible with telephone networks using digital signaling techniques.
It would therefore be desirable to provide automatic voice messaging in a telephone system having various types of switching circuits that are interconnected via digital serial links.